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A personal FM transmitter is a low-power FM radio transmitter that transmits a signal from a portable audio device (such as an MP3 player) to a standard FM radio. Most of these transmitters are connected to the headphone jack of the device and then transmit the signal through an FM transmission band frequency, so that it can be picked up by any nearby radio. This allows portable audio devices to use the highest or best sound quality of a home audio system or car stereo without the need for a wired connection. They are often used in cars but they can also be in fixed locations such as a sound computer card broadcasting through a building.

Being low power, most transmitters typically have a short range of 100-300 feet (30-100 meters), depending on the quality of the receiver, obstructions and elevation. They generally broadcast on any FM frequency of 87.5 to 108.0 MHz in most of the world (or 88.1 to 107.9 MHz in the US and Canada).

Applications

Personal FM transmitters are commonly used as a solution for playing portable audio devices on car radios that do not have an auxiliary "AUX" input connector or Bluetooth audio connectivity. They are also used to transmit a stationary audio source, such as a computer or television, around a house. They can also be used for low-power broadcasting and pirate radio, but only for a very limited audience in the vicinity. It can also be used as a "talking sign" in real estate sales or similar.

Legality

The legality and maximum permitted power levels or field strengths of these devices vary by country. In 2006 these devices became legal in most European Union countries.


In the United Kingdom, Standard Instrument IR2030 / 26/2 2011/0401 / UK (as of December 2011) allows the unlicensed use of devices that can show that they radiate less than 50 nano-watts (-43dBm), in a Raster of 0.2MHz in the range of 87.5-108MHz.
Industry Canada allows transmitters with a power of less than 100 μV / m to 30 meters (approximately 1 micro-watt of power).

In the United States, Part 15 of the US Federal Communications Commission rules that no license is required if FM transmitters have a Maximum Effective Radiant Power (ERP) of 0.01 micro-watts or 250 μV / m Measured at 3 meters. In Japan, no license is required for devices with a signal strength of less than 500 μV / m at 3 meters.